Nuclear receptors are a superfamily of regulatory proteins that are structurally and functionally related and are receptors for, e.g., steroids, retinoids, vitamin D and thyroid hormones (see, e.g., Evans, Science, 240:889-895 (1988)). These proteins bind to cis-acting elements in the promoters of their target genes and modulate gene expression in response to ligands for the receptors.
Nuclear receptors can be classified based on their DNA binding properties (see, e.g., Evans, supra, and Glass, Endocr. Rev., 15:391-407 (1994)). For example, one class of nuclear receptors includes the glucocorticoid, estrogen, androgen, progestin and mineralocorticoid receptors which bind as homodimers to hormone response elements (HREs) organized as inverted repeats (see, e.g., Glass, supra). A second class of receptors, including those activated by retinoic acid, thyroid hormone, vitamin D3, fatty acids/peroxisome proliferators (i.e., peroxisome proliferator activated receptors or PPARs) and ecdysone, bind to HREs as heterodimers with a common partner, the retinoid X receptors (i.e., RXRs, also known as the 9-cis retinoic acid receptors; see, e.g., Levin et al., Nature, 355:359-361 (1992) and Heyman et al., Cell, 68:397-406 (1992)).
RXRs are unique among the nuclear receptors in that they bind DNA as a homodimer and are required as a heterodimeric partner for a number of additional nuclear receptors to bind DNA (see, e.g., Mangelsdorf et al., Cell, 83:841-850 (1995)). The latter receptors, termed the class II nuclear receptor subfamily, include many which are established or implicated as important regulators of gene expression. There are three RXR genes (see, e.g., Mangelsdorf et al., Genes Dev., 6:329-344 (1992)), coding for RXRα, -β, and -γ, all of which are able to heterodimerize with any of the class II receptors, although there appear to be preferences for distinct RXR subtypes by partner receptors in vivo (see, e.g., Chiba et al., Mol. Cell. Biol., 17:3013-3020 (1997)). In the adult liver, RXRα is the most abundant of the three RXRs (see, e.g., Mangelsdorf et al., Genes Dev., 6:329-344 (1992)), suggesting that it might have a prominent role in hepatic functions that involve regulation by class II nuclear receptors. See also, Wan et al., Mol. Cell. Biol., 20:4436-4444 (2000).
LXRα is found predominantly in the liver, with lower levels found in kidney, intestine, spleen and adrenal tissue (see, e.g., Willy et al., Genes Dev., 9(9):1033-1045 (1995)). LXRβ is ubiquitous in mammals and was found in nearly all tissues examined LXRs are activated by certain naturally occurring, oxidized derivatives of cholesterol (see, e.g., Lehmann et al., J. Biol. Chem., 272(6):3137-3140 (1997)). LXRα is activated by oxycholesterol and promotes cholesterol metabolism (Peet et al., Cell, 93:693-704 (1998)). Thus, LXRs appear to play a role in, e.g., cholesterol metabolism (see, e.g., Janowski et al., Nature, 383:728-731 (1996)).
The nuclear receptor LXR plays a critical role in coordinate control of bile acid, cholesterol, and triglyceride metabolism to maintain lipid homeostasis. LXRs and bile acid/oxysterol-regulated genes are potential targets for developing drug therapies for lowering serum cholesterol and treating cardiovascular and liver diseases. Compounds with activity at LXR can have profound effects on lipid homeostasis, and can more effectively control disease or disorders in which LXR is implicated. This is accomplished through regulation of multiple genes involved in cholesterol homeostasis including Cyp7a1, a member of the cytochrome p450 family of enzymes and the rate limiting step in bile acid synthesis, as well as the ABC membrane transporters ABCA1, ABCG1, ABCG5, and ABCG8. ABCA1 is critical in the efflux of cholesterol and phospholipids to lipid-poor lipoproteins such as ApoA-I thus contributing to an increase in plasma HDL levels. In addition, ABCG5 and ABCG8 appear to mediate decreased intestinal absorption of cholesterol and facilitate cholesterol efflux from liver cells into the bile. Unfortunately, in addition to the anti-atherogenic effect of LXR agonists, studies in cell culture and animal model systems have demonstrated that LXR agonists increase plasma triglyceride levels and hepatic lipogenesis and promote the increased production of VLDL lipoprotein particles. Schultz et al., Genes Dev., 14:2831-2838 (2000); Repa et al., Genes Dev., 14:28119-2830 (2000). Strategies to minimize the undesirable lipid effects include identifying LXRβ selective compounds that are also partial agonists. Partial agonists can display tissue-specific activation or repression of nuclear receptors, as was demonstrated for the anti-estrogen tamoxifen, which functions as an antagonist of estrogen signaling in breast tissue and an agonist in the uterus. Characterization of LXR isoform-specific null mice indicate that LXRα is the predominant mediator of LXR activity in the liver. In macrophages, however, LXRβ alone is sufficient to mediate the effects of LXR ligands on target gene expression. Therefore compounds with limited LXRα activity should have anti-atherogenic activity while limiting unwanted hepatic effects.
Nuclear receptor activity has been implicated in a variety of diseases and disorders, including, but not limited to, hypercholesterolemia (see, e.g., PCT Publication No. WO 00/57915), osteoporosis and vitamin deficiency (see, e.g., U.S. Pat. No. 6,316,503), hyperlipoproteinemia (see, e.g., PCT Publication No. WO 01/60818), hypertriglyceridemia, lipodystrophy, hyperglycemia and diabetes mellitus (see, e.g., PCT Publication No. WO 01/82917), atherosclerosis and gallstones (see, e.g., PCT Publication No. WO 00/37077), disorders of the skin and mucous membranes (see, e.g., U.S. Pat. Nos. 6,184,215 and 6,187,814, and PCT Publication No. WO 98/32444), acne (see, e.g., PCT Publication No. WO 00/49992), and cancer, Parkinson's disease and Alzheimer's disease (see, e.g., PCT Publication No. WO 00/17334). Activity of nuclear receptors, including LXRs, FXR and PPAR, and orphan nuclear receptors, has been implicated in physiological processes including, but not limited to, bile acid biosynthesis, cholesterol metabolism or catabolism, and modulation of cholesterol 7.alpha.-hydroxylase gene (CYP7A1) transcription (see, e.g., Chiang et al., J. Biol. Chem., 275:10918-10924 (2000)), HDL metabolism (see, e.g., Urizar et al., J. Biol. Chem., 275:39313-39317 (2000) and PCT Publication No. WO 01/03705), and increased cholesterol efflux and increased expression of ATP binding cassette transporter protein (ABC1) (see, e.g., PCT Publication No. WO 00/78972).
Thus, we recognized that there is a need for compounds, compositions and methods of modulating the activity of the LXR nuclear receptors in ways that separate the desirable effects on cholesterol metabolism and atherogenesis from increased plasma triglyceride levels and an increase in hepatic lipogenesis. Although full agonists of LXR cause both the desirable and undesirable effects, the present invention describes compounds that have a beneficial separation between the two, and thus have an improved therapeutic index between increased reverse cholesterol transport and detrimental effects on plasma triglycerides and LDL-cholesterol.